Infertility and other untoward reproductive and developmental outcomes, such as spontaneous abortion, fetal and neonatal death, birth defects, and genetic susceptibilities to cancer and other diseases, are associated with genetic damage occurring in mammalian germ cells. This Project employs molecular biomarkers in the identification and characterization of chemicals in our environment that are responsible for such germ cell damage. Specifically, fluorescence in situ hybridization (FISH) with chromosome specific DNA markers are used to detect structural and numerical chromosome damage in sperm and early embryonic cells of rodents. These rodent sperm-FISH assays, which provide parallel models of similar sperm-FISH assays in humans, are being used for evaluating the aneugenic and clastogenic potential of environmental chemicals tested in NTP bioassays, as well as for addressing questions about dose response, differential stage sensitivity, the relationship of defects seen in sperm to those transmitted to the early embryo, and other important issues related to health-risks which cannot be addressed in human studies. During this past year, frozen samples of rat sperm were obtained from NTP Reproductive Assessment by Continuous Breeding (RACB) studies on hexachlorobenzene and tetrachloroazobenzeneare, and frozen samples of mouse sperm were obtained from an RACB study of AZT/ddI and from micronucleus studies of benzene, diazoaminobenzene, analine and cyclophosphamide.